Flywheel assembly

ABSTRACT

A flywheel assembly 10 comprising: at least one flywheel mass support 14, the or each said support having a shaft 19 that extends along a rotational axis 18 about which the support 14 can rotate in use, the or each said support 14 comprising a plurality of openings 24 that are each offset from said rotational axis 18, a flywheel mass 12 comprising a plurality of openings 16 that are each arranged to align with a corresponding opening in said support; and means 23 for coupling said flywheel mass 12 to the or each said support 14 so that the mass 12 can rotate with the or each support 14 in use, said coupling means 23 being configured to extend through the aligned openings in the or each support 14 and said flywheel mass 12; wherein said flywheel mass 12 comprises a plurality of generally planar flywheel mass elements sandwiched together to form a stack of elements, each said element including a plurality of openings 16 that align with the openings 24 in the or each said support 14 and with openings in neighbouring elements in said stack, said flywheel elements being coupled together and aligned with one another to form said flywheel mass solely by means of the coupling means 23 that extends through the aligned openings in said elements and the or each said support 14.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/345,630, which is the National Stage of International Application No.PCT/EP2012/068340, filed Sep. 18, 2012. The patent applicationidentified above is incorporated here by reference in its entirety toprovide continuity of disclosure.

FIELD

The present invention relates to a flywheel assembly and to a flywheelmass for use in such an assembly.

BACKGROUND

A previously proposed flywheel assembly comprises a flywheel mass havinga hole extending through it along a rotational axis about which theflywheel mass spins in use. An axle extends through the hole of theflywheel mass such that the axle forms an interference fit with theflywheel mass. In this configuration rotating the axle causes theflywheel mass to rotate about the rotational axis.

Flywheel assemblies like the one described above are used to storeenergy in the form of kinetic energy. For example, the translationalkinetic energy of a car may be transferred to the rotational kineticenergy of an onboard flywheel assembly when the car stops rather thanbeing dissipated as heat in the car's brakes. The rotational kineticenergy may then later be recovered and transferred to translationalkinetic energy of the car by mechanically linking the rotating flywheelassembly to a wheel axle of the car. A flywheel assembly may also beused to store energy generated by an intermittent power source such as asolar powered motor. For example, when such a motor is energised it maybe used to rotate a flywheel assembly such that later on, when the motoris no longer energised, energy may be recovered from the rotatingflywheel assembly by using the flywheel assembly to rotate one or moremagnets for electromagnetic induction purposes (i.e. to generateelectricity).

It will be appreciated that the amount of energy E_(K) capable of beingstored by a rotating flywheel assembly is governed by the equation

$E_{K} = {\frac{1}{2}I\;\omega^{2}}$

wherein l is the moment of inertia of the flywheel assembly and ω is theangular velocity at which the assembly rotates. Therefore, in order toincrease the amount of energy stored by a flywheel assembly it isnecessary to increase the speed at which the assembly rotates. However,if a flywheel mass is rotated at high speed, stresses induced bycentrifugal forces on the flywheel mass of the assembly may cause theflywheel mass to deform thereby leading to various potential problems.

One such problem is that a flywheel mass can disengage from a flushcontact fit with the rotating axle as the mass deforms in a directionaway from the axle. If this should happen, then the flywheel mass maymove relative to the axle in spite of any axial clamping forces, suchmovement likely to be radially to one side. Once this has occurred, theflywheel mass is unbalanced and will tend to create load that can eitherdamage the bearings of the flywheel assembly and/or create unwantedvibrations.

It is also possible that the flywheel mass may slip relative to the axisin the tangential direction. This can also lead to loss of balance butalso relative movement between the flywheel mass and axle. The mass mayslow down and begin to retract until an interference fit is re-formedwith the rotating axle. At the instant the flywheel mass reforms theinterference fit, frictional forces between the flywheel mass and therotating axle apply an impulse to the flywheel mass. The flywheel masswill again pick up speed by further rotation of the axle and thedeformation, disengagement and retraction process will occur once again.However, repeated application of impulses to a flywheel mass increasesthe likelihood of the flywheel mass fracturing in use. As a consequence,there is a limit to the speed at which such flywheel masses can berotated.

One previously proposed arrangement (shown in FIG. 1) for addressingthis problem is described in PCT patent application numberPCT/GB2011/000053. Looking at FIG. 1, this document discloses a flywheelapparatus 1 that comprises a deformable hub 2 which is fixed to an axle3. A plurality of discs 4 are arranged in contact with an exteriorsurface of the hub 2 and a plurality of weights 5 are arranged incontact with an interior surface of the hub 2 to form a flywheel mass.In use, as the axle 3 rotates, so the hub 2 (and thus the discs 4 andweights 5) rotate with the axle. At high rotational speeds the discs 4tend to deform in a direction away from the axle 3 (i.e. radiallyoutwardly), and this would—in the absence of a deformable hub—cause thediscs to separate from the hub. However, in the arrangement described inthis document, centrifugal forces acting on the weights 5 aretransferred to the hub 2 and these forces cause the hub 2 to outwardlydeform so that it remains in contact with the discs 4 and the axle. As aresult, the flywheel masses can be rotated at a higher speed thanpreviously proposed arrangements.

Whilst this design does have the advantage that failure is likely onlyto occur in one disc atone instance as opposed to having a single massthrough which a crack can propagate and cause all of the mass to fail inone go. This may occur if sufficient speed generated stresses induced bycentrifugal forces in the discs 4 exceed a point where the discs 4 canfracture. If this should occur, disc fragments will be ejected from theapparatus at high speed and hence the flywheel mass must be contained ina suitable casing. It will be appreciated that the energy released whenone disc fails is significantly less than that released if the entiremass should fail, and hence the containment casing for this assembly canbe lighter than for an assembly with a single mass.

Unfortunately, one disadvantage associated with such a solution is thatthe cost of manufacturing the flywheel assembly is increased—principallydue to the presence of both a hub 2 and weights 5 in addition to the oneor more discs 4.

It would thus be advantageous if it were possible at a reasonable costto both increase the amount of energy that can be stored by a flywheelassembly and reduce the risk of injury to people in the vicinity of theassembly in the event that the assembly should fail.

Another previously proposed flywheel assembly is disclosed in JapanesePatent Application no. 58215948 (Mitsubishi Electric Corp). In thisassembly, the flywheel mass is comprised of a plurality of individualdisks that are arranged in a stack between a top and a bottom plate. Thedisks between the top and bottom plates each include a locatingprojection on one surface that mates closely with a complementarydepression on an opposing surface of the neighbouring disk in the stack(to thereby align the plates). The top and bottom plates are locallythickened towards the centre of the mass, and axle connecting plates arebolted to the locally thickened parts of the top and bottom disks sothat the mass can be rotated around an axle.

This arrangement has numerous disadvantages. Firstly, the fact that thedisks require closely cooperating projections and depressions means thatthe disks must be carefully machined to have the right shape. Thisincreases the cost of manufacturing the assembly, and precludes thisassembly from manufacture in regions of the world where access to suchmachinery is not readily available. Secondly, drilling into the top andbottom plates to enable the axial hubs to be attached inevitably weakensthe plates, thereby increasing the risk of failure. Thirdly, theinterlocking projections and depressions that align the plates and diskscreate stress points at each discontinuity that inevitably reduce themaximum speed at which the mass can be rotated.

Aspects of the present invention have been devised with the foregoingissues in mind.

SUMMARY

In accordance with one presently preferred implementation of theteachings of the present invention, there is provided a flywheelassembly comprising: at least one flywheel mass support, the or eachsaid support having a shaft that extends along a rotational axis aboutwhich the support can rotate in use, the or each said support comprisinga plurality of openings that are each offset from said rotational axis,a flywheel mass comprising a plurality of openings that are eacharranged to align with a corresponding opening in said support; andmeans for coupling said flywheel mass to the or each said support sothat the mass can rotate with the or each support in use, said couplingmeans being configured to extend through the aligned openings in the oreach support and said flywheel mass.

An advantage of this arrangement is that the flywheel mass can berotated at higher speeds and hence can store higher amounts of kineticenergy than previously proposed flywheels. It is also the case that at agiven angular velocity ω, this flywheel mass is less likely to fracturethan previously proposed arrangements.

In one implementation, the openings in at least one of said mass and theor each support are substantially circular. The openings in at least oneof said mass and the or each support may instead be non-circular.

The openings in said mass and the or each support may have the sameshape. Alternatively, the openings in said mass and the or each supportmay have different shapes that complement one another so that an openingin said mass can be aligned with an opening in the or each said plate.

The openings in at least one of the or each support and said mass maycomprise a primary region and an auxiliary region. The primary andauxiliary regions may have the same or different cross-sectional shape.Centre points of the primary and auxiliary regions may be equidistantfrom said rotational axis.

In one arrangement said primary region may comprise a part-circle havinga first diameter, and said auxiliary region may comprise a part circlehaving a second diameter that is less than said first diameter.

The flywheel mass may comprise a plurality of flywheel elementssandwiched together, each said element including a plurality ofopenings, said flywheel elements being configured so that openings inrespective elements align with one another when the elements aresandwiched together.

The coupling means may comprise a plurality of fixing members, each saidfixing member being configured to extend through an aligned set ofopenings defined by the flywheel elements.

The or each said support may comprise a plate in which said openings areprovided and from which said shaft extends. The plate and said shaft maycomprise a single element, or two discrete elements that are fixedlycoupled one to the other.

In one arrangement the assembly may comprise two supports, each saidsupport comprising a plate from which a shaft extend, said flywheel massbeing sandwiched between the plates of the supports so that the shaftsof each support are aligned with one another and extend along the samerotational axis.

Another aspect of the present invention relates to a flywheel mass foruse in the flywheel assembly described herein, the flywheel massdefining one or more openings that are each offset from a rotationalaxis about which the flywheel element is configured to rotate in use.

A yet further aspect of the invention relates to a flywheel assemblycomprising: first and second flywheel mass supports, each said supporthaving comprising a plate and a shaft that extends from the plate alonga rotational axis about which the support can rotate in use, each saidsupport further comprising a plurality of openings that are each offsetfrom said rotational axis, a flywheel mass comprising a plurality ofopenings that are each arranged to align with corresponding openings ineach of said supports; and means for coupling said flywheel mass to eachsaid support so that the mass is sandwiched between respective plates ofthe supports and the respective shafts of the supports extend inopposite directions along the same rotational axis, said coupling meansbeing configured to extend through the aligned openings in the firstsupport, through the flywheel mass, and through the openings in thesecond support.

In general terms, arrangements embodying the teachings of the inventionhave one or more of the following advantages: (i) a flywheel mass with aplurality of flywheel elements will tend to release a smaller fractionof the flywheel mass in the event of a structural failure of the mass;(ii) a flywheel mass consisting of a plurality of elements aligned andcoupled together at radially outward locations reduces axial stressesthat tend to promote fracture over yielding (plastic deformation); (iii)the design of the openings through the flywheel elements reduces stressconcentrations so that it becomes possible to operate the flywheelassembly to stress levels comparable to those of a design where theflywheel mass has no openings (i.e. stress levels at the openings areequal to or less than those found at the axial centre of the mass); and(iv) the provision of a support where the bolts do not protrude reducesaerodynamic windage induced drag.

Other features, aspects, embodiments and advantages of the teachings ofthe invention will be apparent from the remainder of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the teachings of the present invention, andarrangements embodying those teachings, will hereafter be described byway of illustrative example with reference to the accompanying drawings,in which:

FIG. 1 is a schematic cross sectional view of a prior art flywheelassembly;

FIG. 2 is a schematic perspective view of an envisaged flywheelassembly;

FIG. 3 is a schematic perspective view of a flywheel element comprisingpart of the flywheel assembly shown in FIG. 2;

FIG. 4 is a schematic perspective view of a shaft and plate for couplingto the flywheel element shown in FIG. 3;

FIG. 5 is a schematic representation of one envisaged flywheel opening;

FIG. 6 is a schematic representation of another envisaged flywheelopening;

FIG. 7 is graphical representation of the stress-radius relationship ofa flywheel element rotating at an angular velocity w, the flywheelelement having a circular aperture extending through its centre alongits rotational axis;

FIG. 8 is a graphical representation of the stress-radius relationshipof a flywheel element rotating at an angular velocity ω, the flywheelelement not having a central aperture extending along its rotationalaxis but instead defining four holes that are offset from the rotationalaxis;

FIG. 9 is a graphical representation of the stress-radius relationshipof a flywheel element rotating at an angular velocity ω, the flywheelelement defining four holes of the shape shown in FIG. 6 such holesbeing offset from the rotational axis;

FIG. 10 is a side view of another envisaged flywheel assembly;

FIGS. 11 & 12 are graphical representations of some illustrative ways inwhich a bolt (or bolts) may extended through the openings shown in FIGS.5 & 6;

FIGS. 13(a) to 13(c), there are depicted three schematic representationsof other holes that may be formed through a flywheel mass;

FIG. 14 is a schematic representation of another flywheel assembly;

FIGS. 15 and 16 are schematic cross-sectional representations ofillustrative machine screws;

FIG. 17 shows an alternative arrangement for tensioning the bolts; and

FIG. 18 is a schematic representation of another flywheel assembly.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 is a schematic representation of an envisaged flywheel assembly10 that comprises a flywheel mass 12 that is sandwiched between, andcoupled to, two flywheel mass supports 14.

Looking at FIG. 3 the flywheel mass 12 comprises a body with one or morethrough-holes 16 that are each radially offset from a rotational axis 18about which the flywheel mass 12 spins in use.

As shown in FIG. 4, each flywheel support 14 comprises a plate 20 and anaxial shaft 19 that defines the aforementioned axis 18 about which theplate 20 can rotate. The plate 20 includes two or more through holes 22that are each radially offset, such holes being offset from the shaft19. The holes 22 are positioned so that they are able to align with theholes 16 in the flywheel element 12 when the plate 20 is abutted againstthe flywheel mass 12. In one particular arrangement the holes 22 definedby the plate 20 have cross-sectional shapes that are at leastcomplementary, and in one envisaged arrangement correspond (at leastapproximately), to those of the holes 16 in the flywheel mass 12.

The flywheel assembly 10 is formed by abutting one plate 20 against anupper surface 13 of the flywheel mass 12 and another plate 20 against alower surface 15 of the flywheel mass 12. The holes 22 defined by theplates 20 are then aligned with the holes 16 in the flywheel mass 12before the plates 20 are secured to the flywheel mass 12 by inserting asuitable fixing element 23 (such as a bolt, pin, rod or stub shaft)through the aligned holes 16, 22, and securing the fixing element inplace so that the plates are tightly coupled to the flywheel element 12.Once assembled the flywheel assembly 10 can then be rotated about itsrotational axis 18 to store kinetic energy. In FIG. 10, the fixingelements illustrated in the drawing protrude from the plate and whilstthis arrangement functions adequately, it would advantageous to useelements that project to a lesser extent (preferably, elements that lieflush with the plate) to reduce windage losses (FIG. 14 illustrates oneenvisaged arrangement that reduces windage losses).

As will be appreciated, in this arrangement the plate(s) are coupled tothe mass solely by means of the fixing elements that extend through theplate(s) and the mass, rather than being bolted to the mass (as in theaforementioned Mitsubishi arrangement). This arrangement is advantageousas it avoids the potential for stress-induced failure at the holesdrilled into the mass (which holes engage with the bolts to hold themass to the support).

In one arrangement the flywheel mass 12 includes one pair of holes 16.In this arrangement the holes are (at least roughly) equidistant from,and linearly aligned with respect to, the rotational axis 18. In anotherarrangement the flywheel mass 12 also defines one or more additionalholes 16, each such hole being located the same distance away from therotational axis 18. For example, in the particular arrangement shown inFIG. 3 the flywheel mass 12 includes two pairs of holes and the holes ofeach pair fall on a notional line running through the rotational axis18, and are (at least roughly) equidistant from the rotational axis 18of the flywheel mass 12.

The holes 16 in the flywheel mass 12 (and plate 20) may, in oneenvisaged implementation, be circular in shape. In another arrangement,one or more such holes may instead be non-circular, and optionally bemade up of a plurality of regions. For example, the hole shown in FIG. 5has a primary region 24 of a first cross-sectional shape and a firstpair of auxiliary regions 26 each of which has a second cross sectionalshape (which may or may not be the same as the first cross-sectionalshape). In the particular arrangement shown in FIG. 5, the primaryregion 24 comprises a part-circle having a first diameter d₁, and theauxiliary regions 26 comprise part-circles with a second diameter d₂that is smaller than that of the primary hole region (and preferablyequal to one another). As shown in FIG. 5, the auxiliary regions 26 arelocated to either side of the primary region and open into the primaryregion 24. In a preferred implementation, at least the centres “O” ofthe part-circles that form the auxiliary regions are equidistant fromthe axis of the plate/flywheel mass and hence arranged along a curve 28.In the particular arrangement shown in FIG. 5, the part-circles thatform the primary and auxiliary regions are arranged along the samecurve.

Referring now to FIG. 6, it is also envisaged that a hole may furthercomprise one or more additional pairs of auxiliary regions (such as thesecond pair of auxiliary opening portions 30). In the particulararrangement depicted in FIG. 6, the part-circles that define the primaryand each auxiliary region are all equidistant from the axis of theplate/flywheel element and hence arranged along the same curve 28. Inother envisaged arrangements, the centres of the part-circles thatcomprise the second auxiliary openings may be located along a curvehaving a different radius to that of the curve along which thepart-circles that comprise the primary auxiliary openings are located,and the curve on which the primary region is located may have the sameradius or a different radius to that of the first or second auxiliaryregions.

In one envisaged implementation the plates and the flywheel mass haveholes that are the same shape. It will be appreciated, however, thatthis is not essential and that it will suffice if the plates andflywheel mass have holes that complement each other. For example, in oneenvisaged arrangement the mass may have a plurality of through holes ofthe type depicted in FIG. 5, and the plates may comprise a plurality ofcircular holes each arranged to align with one of the primary 24 orauxiliary 26 regions of a flywheel hole. It will also be appreciatedthat it is not essential for the holes to comprise a pair of auxiliaryregions; they could instead comprise a single primary region and asingle auxiliary region. Yet further arrangements will be apparent topersons of ordinary skill in the art.

FIGS. 7 to 9 illustrate the relationship between i) the radius of aflywheel element and ii) the tangential stress on the mass that iscreated by centrifugal forces when such a flywheel mass is rotated at anangular velocity ω.

FIG. 7 illustrates the stress-radius relationship of a flywheel massrotating at an angular velocity ω, the flywheel mass having a constantthickness and a circular aperture extending through its centre along itsrotational axis. As shown in FIG. 7, this flywheel mass experiences thehighest amount of stress from centrifugal forces in use at the innermostsurface of the flywheel mass (at radius A) i.e. the surface defining theaperture through the centre of the flywheel mass. In contrast, theoutermost surface of the flywheel mass (at radius B) experiences theleast amount of stress from centrifugal forces.

FIG. 8 illustrates the stress-radius relationship of a flywheel massrotating at an angular velocity ω, the flywheel mass having a constantthickness and no central aperture extending along its rotational axisbut instead defining four holes that are offset from the rotationalaxis. As shown in FIG. 8, this flywheel mass experiences the highestamount of stress from centrifugal forces in use at the innermost surfaceof each hole (at radius D), i.e. that surface closest to the rotationalaxis (at radius O). The outermost surface of each hole (at radius E),i.e. that surface furthest from the rotational axis (at radius O),experiences slightly less stress from centrifugal forces than theinnermost surface (at radius D). The outermost surface of the flywheelmass (at radius B) experiences the least amount of stress fromcentrifugal forces in use.

FIG. 9 illustrates the stress-radius relationship of a flywheel massrotating at an angular velocity ω, the flywheel mass having a constantthickness and defining four holes of the shape shown in FIG. 6, suchholes being offset from the rotational axis. As shown in FIG. 9, thisflywheel mass experiences the highest amount of stress from centrifugalforces in use at the innermost surface of each hole (at radius O) i.e.that surface closest to the rotational axis (at radius O). The outermostsurface of each hole (at radius E), i.e. that surface furthest from therotational axis (at radius O), experiences slightly less stress fromcentrifugal forces than the innermost surface (at radius D). Theoutermost surface of the flywheel mass (at radius B) experiences theleast amount of stress from centrifugal forces in use. Since the maximumstress at the aperture (at radius D) is lower than the stress at thedisc centre (at radius O), the maximum stress for this geometry is thesame as a plane disc without any apertures. It follows, therefore, thatsuch a disc with apertures, creating a useful machine, can operate atthe same speed and store the same energy as a disc without apertures.

As shown in FIGS. 7 to 9, a flywheel mass having an aperture along itsrotational axis experiences the highest amounts of stress fromcentrifugal forces when rotated at angular velocity ω. Such flywheelmasses are thus likely to fracture at slower rotational speeds thanflywheel masses of the type shown in FIGS. 8 & 9. One consequence ofthis is that flywheel masses of the type shown in FIGS. 8 & 9 can berotated at higher speeds and hence can store higher amounts of kineticenergy than those of the type shown in FIG. 7. It is also the case thatat a given angular velocity ω, the flywheel masses shown in FIGS. 8 and9 are less likely to fracture than the mass depicted in FIG. 7.

In another envisaged arrangement (depicted schematically in FIG. 10) theflywheel mass 12 of the fly wheel assembly 10 may comprise a pluralityof generally planar flywheel elements 15 stacked on top of one another.In such an arrangement plates 20 are engaged with the upper and lowerflywheel elements 15 such that the shafts 14 coupled thereto extendalong the rotational axis of the flywheel elements. To secure theassembly together, the holes 16 defined by the respective flywheelelements 15 and the holes 22 defined by the plates 20 are aligned suchthat a fixing element 23 (for example a bolt, pin, rod or stub shaft)may be extended through each aligned set of holes and secured in place,thereby coupling the flywheel elements 12, plates 20 and shafts 19together. As will be appreciated by persons of ordinary skill in theart, the elements are located with respect to one another solely bymeans of the bolts that extend through the openings in the elements, andhence require no complex machining of the type required to form thecomplementary steps and recesses of the aforementioned Mitsubishiarrangement that function to align the elements with respect to oneanother. The arrangement presently employed also avoids the stressconcentrations that occur at the step/recess interfaces of theMitsubishi arrangement.

An advantage of this arrangement is that a flywheel consisting of a setof flywheel elements (which release less energy in the event of aflywheel element fracture than a flywheel made from a single mass) canbe operated at the same speed as a disc without apertures. This reducesthe weight and cost of the containment system which is required toreduce the likelihood of mass parts being ejected in the event of aflywheel mass failure. Additionally, the cost of manufacture andassembly of such a design is further reduced since the flywheel elementsare generally planar and can therefore be stamped (or otherwise formed)from sheet metal material with little additional processing to form themajority of the flywheel assembly. This makes such a flywheel designattractive for use in automotive energy recovery systems and groundbased power applications particularly as an alternative to compositesstructures. The resulting flywheel also operates at a lower peripheralspeed than a composite flywheel for a given amount of energy stored, andhence windage losses are reduced.

In the case of application of this invention to developing countries asa means of ground power storage, one advantage of this arrangement isthat if one or more elements should be damaged, then only those elementsneed to be replaced. Assembly and repair of this design thereforesimpler than other designs and can be done in workshops with morelimited specialist equipment.

Various ways of using a fixing element 23 to secure a plate 20 and oneor more flywheel elements 12 together are shown in FIGS. 11 & 12. Forexample, a fixing element 24 may be extended through only one of theopening portions of a hole. Alternatively, fixing elements 24 may beextended through two or more of the opening portions of a hole. Anadvantage of the arrangements proposed is that a variety of differentsized fixings can be used to assemble the flywheel assembly.

Referring now FIGS. 13(a) to 13(c), there are depicted three schematicrepresentations of other holes that may be formed through the flywheelmass. FIG. 13(a) is a schematic view of a hole 32 through the flywheelmass that has a predominantly elliptical shape in order to reduce stressin the discs that make up the flywheel mass. A bolt 34 passes throughthe hole 32, and inserts 36 have been added between the bolt and thehole in order to stabilise the bolt position. In one envisagedarrangement the inserts may be made from a low density material such ashard polymer or light metal alloy. In this particular arrangement, thehole 32 is (at least generally) symmetrical but it can be beneficial tomake the shape asymmetric to further reduce stress as shown in FIG.13(b). In another arrangement, the two inserts per hole may be replacedwith one insert 38 by reducing the size of the bolt 34. This arrangementmakes assembly easier, but may increase the stress by virtue of theincreased size of the hole.

Referring now to FIG. 14, there is depicted a schematic representationof another flywheel assembly 40. The flywheel assembly of thisembodiment is similar in many respects to those of other embodiments,but differs in that in this embodiment the plates 20 are configured sothat the bolts do not protrude from the outer surface of the plate(thereby reducing windage losses).

The discs 12 that make up the flywheel mass and the plates 20 areclamped together by bolts 42, 44 prior to operation of the flywheelassembly 40, and as before the plates 20 function to connect theflywheel mass to the shafts 19. The plates 20 can be of a more complexdesign than the flywheel discs since only two plates are required inthis embodiment. The plates are configured to reduce stresses in theflywheel mass and also to support the bolts 42, 44.

In one envisaged arrangement, the bolts 42, 44 each consist of twocountersunk machine screws 46 that are internally threaded, and withwhich an outwardly threaded rod 48 may be engaged. By providinginternally threaded screws, the discs of the flywheel mass locate on therelatively smooth outside surface of the machine screws thereby avoidinglocating on external threads which would be inaccurate. In a preferredarrangement a small gap 50 exists between the screws 46, so that themiddle disc of the flywheel mass is still located without losing thedamping force provided by the bolts 42, 44.

FIGS. 15 and 16 are schematic cross-sectional representations ofillustrative machine screws. In FIG. 15, the internally threaded machinescrew is shown connected to part of the rod 48. Tension is created inthe bolt by applying torque to both of the machine screws against eachother when looking down the axis of the bolt. In FIG. 16 there isdepicted an alternative design of machine screw. In this arrangement therod 48 can penetrate right through the machine screw in order to avoid ablind hole. In this arrangement, a feature on the machine screw (such asa hexagonal socket) for applying torque must be larger than the threadedhole through the screw.

FIG. 17 shows an alternative arrangement for tensioning the bolts. Inthis arrangement the rod 48 protrudes beyond the machine screw 46 on oneside and a nut 48 is threaded onto the protruding thread. Mechanicalapparatus (the like of which is known in the art) is used to pull thenut in the direction of the longer arrows whilst the apparatus pressesagainst plate 20. Such apparatus is well known to one skilled in the artand has apertures so that the machine screw can be turned by engaging atool with locating features 50 formed in the screw 46. Little torqueneeds to be applied to the locating features 50 because once themechanical apparatus releases the force, significant tension is createdin the bolts. After this operation, the protruding rod can be removed bycutting or other form of machining. By connecting the bolts together inthis way, the correct level of tension may be applied to the bolt andthe finished flywheel assembly is inherently less easy to tamper with.

FIG. 18 is a schematic representation of another arrangement where theflywheel is relatively short. In this arrangement bearing support forthe flywheel is only required on one side, and by virtue of thisarrangement one of the plates can be lightened with the shaft normallyrequired for bearing location removed.

It will be appreciated that whilst various aspects and embodiments ofthe present invention have heretofore been described, the scope of thepresent invention is not limited to the particular arrangements set outherein and instead extends to encompass all arrangements, andmodifications and alterations thereto, which fall within the scope ofthe appended claims.

For example, in one envisaged arrangement one or more shafts 19 may besecured directly to the flywheel element 20 in FIG. 3 such that the oreach shaft 19 extends along the length of the rotational axis 18.Furthermore, it is also envisaged that the holes through the flywheelmass and plates need not necessarily be made from sets of circular holesalthough these are simple to create by means of drilling operations. Noncircular holes can be used to reduce stresses in the aperture furtherbut such holes require more complex machinery to create. Creatingapertures of this type can be done using water jet, laser cutting orstamping.

In another envisaged arrangement, the flywheel mass need not necessarilybe sandwiched between a pair of supports but could instead be coupled toa single support. For example, if the support were to be verticallyorientated (so that the flywheel mass rotates around a vertical axis), asecond support plate may be unnecessary. A second support may also beunnecessary if means, other than a second support, are provided tolevitate the flywheel mass. For example, the flywheel mass may compriseone or more magnets, and a housing within which the flywheel massrotates may comprise a further magnet, the magnet on the housing and themagnet(s) on the flywheel mass being arranged so that like poles faceone another.

It should also be noted that whilst the accompanying claims set outparticular combinations of features described herein, the scope of thepresent invention is not limited to the particular combinationshereafter claimed, but instead extends to encompass any combination offeatures herein disclosed.

1.-18. (canceled)
 19. A flywheel mass for an energy storage flywheelassembly, comprising: a plurality of generally planar flywheel masselements forming a stack of flywheel mass elements, each one of theplurality of flywheel mass elements comprising a plurality of openingsthat align with the plurality of openings in adjacent ones of theplurality of flywheel mass elements in the stack of flywheel masselements, wherein the plurality of flywheel mass elements is coupledtogether and aligned with one another to form said flywheel mass bymeans of a plurality of couplings that extend through an aligned set ofthe plurality of openings in said flywheel mass elements, wherein eachopening of the plurality of openings in said flywheel mass elements hasa predominantly elliptical shape.
 20. The flywheel mass according toclaim 19, further comprising: at least one insert per opening of theplurality of openings, arranged around the respective coupling extendingthrough the aligned set of the plurality of openings in said flywheelmass elements.
 21. The flywheel mass according to claim 20, wherein theinserts are made from a low density material such as hard polymer orlight metal alloy.
 22. The flywheel mass according to claim 20, whereineach opening is larger than a diameter of each coupling, so that theflywheel mass comprises only one insert per opening of the plurality ofopenings, which surrounds the respective coupling.
 23. The flywheel massaccording to claim 19, wherein the predominantly elliptical shape issymmetrical or asymmetrical.
 24. The flywheel mass according to claim19, wherein a first semi-minor axis of the predominantly ellipticalshape from a center to a first co-vertex is longer than a secondsemi-minor axis from the center to a second co-vertex.
 25. The flywheelmass according to claim 19, wherein a major axis of the shape,connecting vertexes of the shape and a center of the shape, is a curvedmajor axis, the curved major axis having a turning point on the centerof the shape.
 26. An energy storage flywheel assembly, comprising: atleast one flywheel mass support having a shaft extending along arotational axis about which the at least one flywheel mass support isrotatable, the at least one flywheel mass support comprising a firstplurality of openings, each one of the first plurality of openings beingoffset from said rotational axis; a flywheel mass comprising a pluralityof generally planar flywheel mass elements forming a stack of flywheelmass elements, each one of the plurality of flywheel mass elementscomprising a second plurality of openings that align with the secondplurality of openings in adjacent ones of the plurality of flywheel masselements in the stack of flywheel mass elements, each one of the secondplurality of openings being arranged to align with a corresponding oneof the first plurality of openings of the at least one flywheel masssupport; and a plurality of couplings for coupling said flywheel mass tothe at least one flywheel mass support so that the flywheel mass isrotatable with the at least one flywheel mass support about therotational axis, said plurality of couplings being configured to extendthrough an aligned set of the first plurality of openings in the atleast one flywheel mass support and the second plurality of openings insaid flywheel mass elements, the plurality of flywheel mass elementsbeing coupled together and aligned with one another to form saidflywheel mass solely by means of the plurality of couplings that extendthrough the aligned of the first plurality of openings in the at leastone flywheel mass support and the second plurality of openings in saidflywheel mass elements, wherein each opening of the second plurality ofopenings in said flywheel mass elements has a predominantly ellipticalshape.
 27. The energy storage flywheel assembly according to claim 26,further comprising: at least one insert per opening of the secondplurality of openings, arranged around the respective coupling extendingthrough the aligned set of the first plurality of openings in the atleast one flywheel mass support and the second plurality of openings insaid flywheel mass elements, optionally wherein the inserts are madefrom a low density material such as hard polymer or light metal alloy.28. The energy storage flywheel assembly according to claim 26, whereineach opening of the second plurality of openings is larger than thecouplings, so that the flywheel mass comprises only one insert peropening of the second plurality of openings, which surrounds therespective coupling.
 29. The energy storage flywheel assembly accordingto claim 26, wherein the predominantly elliptical shape is symmetricalor asymmetrical.
 30. The energy storage flywheel assembly according toclaim 26, wherein the at least one flywheel mass support comprises aplate from which the shaft extends and in which the first plurality ofopenings is provided, optionally wherein said plate and said shaftcomprise a single component, or two separate components that are fixedlycoupled to one another.
 31. The energy storage flywheel assemblyaccording to claim 30, comprising two flywheel mass supports, whereinthe flywheel mass is attached to the two flywheel mass supports so thatthe flywheel mass is sandwiched between the plates of the two flywheelmass supports and so that the shafts of the two flywheel mass supportsextend in opposite directions along the rotational axis.
 32. The energystorage flywheel assembly according to claim 26, wherein said pluralityof couplings comprises a plurality of fixing members, each one of theplurality of fixing members configured to extend through the aligned setof openings of the second plurality of openings defined by the flywheelmass elements.
 33. The energy storage flywheel assembly according toclaim 26, wherein the at least one flywheel mass support is fixed to theflywheel mass only by means of the plurality of couplings that extendthrough the at least one flywheel mass support and the flywheel mass.34. The energy storage flywheel assembly according to claim 26, whereinthe at least one flywheel mass support comprises a support body, saidsupport body having a recess surrounding each one of said firstplurality of openings so that the plurality of couplings does notproject from the support body of the at least one flywheel mass supportwhen the plurality of couplings couple the at least one flywheel masssupport to the flywheel mass.
 35. A flywheel mass for an energy storageflywheel assembly, comprising: a plurality of generally planar flywheelmass elements forming a stack of flywheel mass elements, each one of theplurality of flywheel mass elements comprising a plurality of openingsthat align with the plurality of openings in adjacent ones of theplurality of flywheel mass elements in the stack of flywheel masselements, wherein the plurality of flywheel mass elements is coupledtogether and aligned with one another to form said flywheel mass bymeans of a plurality of couplings that extend through an aligned set ofthe plurality of openings in said flywheel mass elements, wherein eachopening of the plurality of openings in said flywheel mass elements hasa non-circular shape.
 36. The flywheel mass according to claim 35,wherein the non-circular shape approximates an ellipse.
 37. The flywheelmass assembly according to claim 36, wherein the non-circular shapeapproximating an ellipse is asymmetrical.
 38. The flywheel massaccording to claim 35, wherein: the non-circular shape is a curvedellipse, in which: a first semi-minor axis from a center of the shape toa first co-vertex is longer than a second semi-minor axis from thecenter of the shape to a second co-vertex; and/or a major axis of theshape, connecting vertexes of the shape and a center of the shape, is acurved major axis, the curved major axis having a turning point on thecenter.